KrF excimer lasers are rapidly becoming the most important light source for integrated circuit lithography. Units currently being sold typically operate at a pulse repetition rate of about 1000 Hz, producing 10 mJ pulses at a wavelength of about 0.248 micron with a bandwidth of about 0.5 m.
Although these lasers are very complicated machines, their reliability has greatly improved during the past few years, and they are currently being integrated into fullscale integrated circuit production.
A detailed description of a KrF laser system is described in U.S. Pat. No. 4,959,840 issued Sep. 25, 1990 (incorporated herein by reference), and assigned to Applicant's employer. As explained in that patent, the excimer gain medium is produced by electric discharges between two elongated electrodes in a flowing gas medium which may be a combination of Krypton, fluorine and a buffer gas, neon. The proportions are typically 0.1 percent fluorine, 1.0 percent krypton and the rest neon. The operating pressure is about 3 atmospheres. The partial pressure of the fluorine is in the range of about 25 kPa of 1 percent F.sub.2 in a buffer gas, such as Neon.
It is known that within the normal operating range of the KrF laser, output pulse energy can be increased by increasing the pulse discharge voltage; and it can be increased by increasing the fluorine concentration. Increases or decreases in both or either of these parameters is easily accomplished with these narrow band KrF excimer lasers.
Fluorine gas is extremely reactive, and in spite of great efforts to utilize materials which are compatible with fluorine, reactions do occur in the chamber depleting the fluorine, especially during and immediately following the electrical discharges during which time the fluorine is ionized.
A typical operating plan for producing constant laser pulses is to compensate for the fluorine depletion by increases in the discharge voltage. This is accomplished with a feedback control which monitors pulse energy on a "per pulse" basis at pulse frequencies such as 1,000 Hz and controls the voltage to maintain substantially constant pulse energy as the fluorine concentration decreases over time. Normally the operating plan will encompass a voltage control range so that when the voltage increases to compensate for the depleted fluorine, reaches an "upper limit" (usually requiring a period of about two hours), a quantity of fluorine is injected during a period of a few seconds. The quantity injected is predetermined to correspond to roughly to the quantity which would have been depleted over the two-hour period. During the fluorine injection period, the automatic feed back control will force the voltage down in order to keep pulse energy substantially constant so that at the end of the injection period the voltage is approximately at the low level of the voltage operating range and fluorine pressure is approximately at its high level. During the next two hours, the process will repeat, and this general process may continue for several days. FIG. 1 shows a graph of average voltage as a function of pulse count for an operating unit. Note that at a pulse rate of 1000 Hz, 1 million pulses correspond to about 16 minutes, and that for continuous operation, the injection period would be at intervals of about 1.3 hours, corresponding to about 5,000,000 pulses. (Often these lasers run at a duty factor of about 20-60 percent. Typically, the lasers do not operate when the lithography tool is changing positions, which would increase the time interval between injections to about several hours.)
Typical KrF lasers have a fairly broad possible range of operation within which the desired pulse energy can be achieved. For example, in one such laser the charging voltage range is from 567 volts to 790 volts, with the corresponding fluorine pressure range being 36.5 kPa to 18.5 kPa. The charging voltage produces a discharge voltage between the laser electrodes which is in the range of about 14,000 volts to about 20,000 volts and approximately proportional to the charging voltage. This high discharge voltage is provided through the operation of a magnetic pulse compression circuit such as is described in U.S. Pat. No. 5,142,166.
A lithographer may choose any operating range of about 40 volts within the 123-volt-charging voltage range. Prior art procedures for selecting the range were not well thought out. One manufacturer has recommended operation at 75% of the maximum voltage. Another suggested the range be determined based on pulse energy transfer efficiency. Choosing a range that is solely determined by a maximum energy transfer may severely limit the operating range of the laser. In such a case, the operating range would be based on the input voltage, pulsed power capacitances and inductances, gas pressure and gas mixture that would result in the maximum energy being transferred to the discharge from the stored energy in the pulse power system. Since the gas mixture changes (as the laser operates), the transfer of energy to the discharge changes. A simple technique to overcome this is to maintain fluorine relatively constant by injecting frequently. However, such frequent injections is, in many applications, not practical. Also; as described in U.S. Pat. No. 5,142,166; by using energy recovery circuits, even if the laser is not operated in maximum energy transfer regime, the residue energy in the circuit can be recovered for the subsequent pulse. Choosing the correct range can be important because both the operating life of the laser is adversely affected by increased fluorine concentration and also by increased discharge voltage. What is needed is a better process of selecting an operating range for narrow band KrF excimer lasers.